Intrinsic SiO_x-based unipolar resistive switching memory. Ⅱ. Thermal effects on charge transport and characterization of multilevel programing

摘要

Multilevel programing and charge transport characteristics of intrinsic SiO_x-based resistive switching memory are investigated using TaN/SiO_x~(++)Si (MIS) and TiW/SiO_x/TiW (MIM) device structures. Current transport characteristics of high- and low-resistance states (HRS and LRS) are studied in both device structures during multilevel operation. Analysis of device thermal response demonstrates that the effective electron energy barrier is strongly dependent on the resistance of the programed state, with estimates of 0.1 eV in the LRS and 0.6 eV in the HRS. Linear data fitting and conductance analyses indicate Poole-Frenkel emission or hopping conductance in the low-voltage region, whereas Fowler-Nordheim (F-N) or trap-assisted tunneling (TAT) is indicated at moderate voltage. Characterizations using hopping transport lead to hopping distance estimates of ～1 nm in the LRS for both device structures. Relative permittivity values (ε_r were extracted using the Poole-Frenkel formulism and estimates of local filament temperature, where ε_r values were ～80 in the LRS and ～4 in the HRS, suggesting a strongly polarized medium in the LRS. The onset of F-N tunneling or TAT corresponds to an observed "overshoot" in the Ⅰ-Ⅴ response with an estimated threshold of 1.6 ± 0.2 V, in good agreement with reported electroluminescence results for LRS devices. Resistive switching is discussed in terms of electrochemical reactions between common SiO_2 defects, and specific defect energy levels are assigned to the dominant transitions in the Ⅰ-Ⅴ response. The overshoot response in the LRS is consistent with TAT through either the Eγ' oxygen vacancy or the hydrogen bridge defect, both of which are reported to have an effective bandgap of 1.7 eV. The SET threshold at ～2.5 V is modeled as hydrogen release from the (Si-H)_2 defect to generate the hydrogen bridge, and the RESET transition is modeled as an electrochemical reaction that re-forms (SiH)_2. The results provide further insights into charge transport and help identify potential switching mechanisms in SiO_x-based unipolar resistive switching memory.